1. Field of the Invention
The invention is related to the field of optical communication networks, and in particular, to controlling the temperature of a dispersion compensating fiber in an optical amplifier system to reduce polarization mode dispersion (PMD) fluctuation.
2. Statement of the Problem
Many communication companies use an optical network for transmitting data because of its high-bandwidth capacity. Fiber optic cables in the optical network reliably transport optical signals over long distances between a transmitter and a receiver. The fiber optic cables are comprised of transmission fiber, such as a single mode fiber (SMF). Over the length of SMF, the optical signals experience some degradation due to attenuation along the fiber. Fiber background loss in the fiber causes the attenuation, typically about 0.2 dB/km. The optical signals also degrade due to other limitations, such as chromatic dispersion, Polarization Mode Dispersion (PMD), and nonlinear effects.
Optical amplifiers may recover the signal strength of the optical signals. However, optical amplifiers can only recover the power of the optical signals. Distortions caused by chromatic dispersion (CD) and PMD can be recovered by using some form of CD compensation technique and PMD compensation technique, respectively.
In optical networks having bit rates 10 Gb/s and higher, PMD and chromatic dispersion are two major sources of distortion for optical signals. Typically, optical amplifiers use dispersion compensation fiber (DCF) or a dispersion compensation module (DCM) to compensate for chromatic dispersion. DCF has a negative dispersion characteristic that can be used to compensate the positive dispersion from SMF. Some DCMs include a span of dispersion compensating fiber wound around a spool and set in an adhesive for support. The length of the dispersion compensating fiber in the DCM depends on the length of transmission fiber preceding the DCM in the optical network, and the amount of compensation needed. For instance, assume that chromatic dispersion on a SMF is 17 ps/nm per kilometer. For a 100 km span of SMF, the chromatic dispersion is about 1700 ps/nm. A DCF with about −1700 ps/nm at 1550 nm wavelength would be required. The length of DCF for −1700 ps/nm is typically about 10 km long.
When compensating for chromatic dispersion, the bit rate of the optical network is also a concern. As the bit rates increase for an optical network, the tolerance for chromatic dispersion compensation decreases. For instance, for a 10 Gb/s bit rate, the tolerance for chromatic dispersion compensation may be +/−800 ps/nm. For a 40 Gb/s bit rate, the tolerance for chromatic dispersion compensation may be +/−25 ps/nm. Thus, as the bit rates increase in optical networks, chromatic dispersion compensation needs to be more precise.
The following examples illustrate optical amplifiers that include DCF to compensate for chromatic dispersion. One example is an Erbium-doped Fiber Amplifier (EDFA). EDFAs may be single stage, 2-stage, 3-stage, etc. A 2-stage EDFA includes a first stage comprising a span of Erbium-doped fiber and one or more pumps, and a second stage comprising a span of Erbium-doped fiber and one or more pumps. DCF connects between the first and second stage. The DCF connects to the output of the first stage and the input of the second stage of the amplifier. In operation, the span of Erbium-doped fiber in the first stage receives optical signals to be amplified. The pumps in the first stage pump the Erbium-doped fiber as the optical signals travel over the Erbium-doped fiber. The Erbium-doped fiber absorbs the pumped lasers and generates a gain in the optical signals. The gain is generally between 20 dB and 30 dB. The optical signals then pass through the DCF. Due to the inherent properties of DCF, the DCF helps compensate for chromatic dispersion in the optical signals. The DCF insertion loss, typically about 8 dB to 10 dB, reduces the strength of the output signals. Therefore, the second stage operates similar to the first stage to compensate for the gain lost due to the DCF.
Another example is a discrete Raman amplifier. A discrete Raman amplifier can includes a span of DCF and one or more pumps. For discrete Raman amplifiers, the DCF comprises the gain medium to provide gain when pumped. In operation, the DCF span receives optical signals to be amplified. The pumps in the discrete Raman amplifier pump the DCF span as the optical signals travel over the DCF span. The DCF absorbs the pumped lasers and generates a gain in the optical signals due to the Raman Effect. Also, due to the inherent properties of the DCF, the DCF span helps compensate for chromatic dispersion in the optical signals. The gain range of the discrete Raman amplifier is flexible and depends on the wavelength of the pump laser. The pump laser amplifies wavelengths at one Raman Stokes away from the laser wavelength. A first order Raman Stokes comprises the wavelengths about 100 nm longer than the pump laser wavelength in glass fiber. For instance, a 1455 nm pump laser wavelength amplifies optical signals having wavelengths around 1550 nm.
One problem with using DCF to compensate for chromatic dispersion in amplifiers is that DCF has high PMD and thus may be susceptible to Polarization Mode Dispersion (PMD) fluctuation. The amount of PMD in DCF can be greater than the PMD equal to one span of 100 km SMF. PMD is a dynamic pulse broadening phenomena. In a single mode fiber, optical pulses propagating down the fiber may separate into two orthogonal modes of polarization that travel at different speeds. The relative amplitudes of these two pulses are determined by the state of polarization of the input pulse relative to the fiber's input principal states of polarization (PSP). The separation into the two orthogonal modes may be caused by intrinsic and extrinsic factors. The intrinsic factors may result from fiber manufacturing processes, such as core ellipticity, or built-in asymmetric stresses. The extrinsic factors may be caused by stresses due to twisting, bending, and environmental effects, such as temperature and thermal gradients.
If the core of the fiber has a perfectly circular cross-section, then both modes travel at the same speed over the same distance. Otherwise, one mode travels slower than the other resulting in a difference in group velocities (an effect called birefringence). Like chromatic dispersion, the difference in velocities between polarization modes is wavelength dependent. For PMD, the difference in velocity is also time dependent. The difference in propagation time, Δτ, experienced by the two polarization modes at a given wavelength is referred to as the differential group delay (DGD) with units in picoseconds (ps). When the DGD in a fiber becomes excessively large, the receiver is unable to distinguish between a zero bit and a one bit, and bit errors occur eventually resulting in a PMD-induced outage.
Temperature variations can cause induced stress on fiber. The stress may cause PMD to fluctuate rapidly, which may increase the outage probability in high speed optical networks. If PMD fluctuates slowly about its mean value, such as day to night, then to compensate any large PMD variation can be easy. If PMD fluctuates quickly, such as over minutes or less, then PMD compensation is harder and PMD compensators may have to adaptively track and compensate for PMD. PMD may fluctuate on a time scale of minutes, seconds, or milliseconds depending on conditions of the entire optical network. Unfortunately, some current optical network elements may not be adequately protected against temperature variations. If the speed of the PMD fluctuation is determined by the PMD fluctuation of the outside (installed) fiber plant, then the effect of temperature on network elements need not be considered. However, in a real network even some fluctuation of the network elements, such as the DCF, can dominate the speed to the PMD within the network.
Some of the buildings that house components of an optical network are kept at a relatively constant temperature. For instance, optical amplifiers are often installed in an optical network at a repeater hut. The repeater hut is a little building where the fiber comes out of the ground and amplifiers and other components attach to the fiber. The repeater huts often have a heater and/or air conditioner to control the temperature in the repeater hut. A heater in the repeater hut has the purpose of heating the entire hut. If a door or window is open, if the heater malfunctions or is shut off, etc, then the temperature of the hut may change. A temperature change in the hut may result in PMD fluctuation on the optical network. Unfortunately, the heater in a repeater hut is not focused on maintaining a temperature of the components of the optical amplifier. The heater is focused mainly on maintaining a room-temperature environment.
Some EDFAs have been designed to protect against temperature variations. These EDFAs have a heating element around the Erbium-doped fiber span that keeps the Erbium-doped fiber span around 70 degrees Celsius. Erbium-doped fibers without the heating element may generate a gain that tilts over the gain bandwidth. The tilt may be 0.5 dB from one end of the gain bandwidth to the other. In a Wavelength Division Multiplexed (WDM) network having many channels, this gain tilt may not be acceptable. The heating element helps level the gain across the gain bandwidth of the EDFA. The EDFAs are not configured to reduce PMD fluctuation by protecting against temperature variations.
Unfortunately, optical amplifiers have not been effectively configured to protect against temperature variations. An optical network that is subjected to temperature variations may be susceptible to PMD fluctuation. PMD compensation may be difficult task in an optical network that has high and/or fast PMD fluctuation.